This invention relates to a method of preparing controlled release fertilizers containing water soluble nutrients which are easily leached from the fertilizers by moisture in the soil. It is particularly useful for developing a coating on the fertilizers for retarding the release of nutrients such as potassium and nitrogen which are present in the type of fertilizers containing coated soluble potassium salts and urea. The premature loss of such nutrients through leaching undesirably results in needlessly high application rates of fertilizer with consequent unnecessary pollution of ground waters.
The release of nutrients such as water soluble potassium salts and nitrogen has generally been controlled by coating potassium-containing pellets, urea prills and the like with cement. See, e.g., U.S. Pat. Nos. 3,630,713; 3,647,4l6; 4,023,955 and 4,486,217. U.S. Pat. No. 4,486,217 discloses other fertilizer compositions admixed with a phosphate or silicophosphate gel-sol mixture. U.S. Pat. No. 4,082,533 discloses other more expensive fertilizers which are sulfur coated or plastic coated.
Although the prior art coatings have been generally effective in retarding the release of nutrients, the art has been continually seeking ever more effective and economical coatings.